Begin by educating yourself through books like The House Hacking Strategy and podcasts like BiggerPockets, and join online communities for support.

I have a rental property I put on the market for rent, do you guys have or know where to get a yearly lease agreement I can use in Phoenix, AZ? 

It lacks organization, members are logged into WhatsApp and the community is expected to support one another.

can you store dumpsters on site?

Yes, operations is a big part of that, especially for anyone that wants to be a business for the long-term.If I were you, I would be talking to office brokers about partnering more than an online forum.   

I've got about $525K in equity in the property, but lenders have told me that I cannot get a HELOC to access that equity for additional purchases because the duplex was financed as an investment - they have said TX law dictates that HELOC can only be used on primary residences.

Just a builders risk policy if you are doing an extensive rehab on the property and then a landlord dwelling policy when the property is rented.

They will just wait for you to cancel the contract and offer on the listing themselves. 

What is the cashflow like on those new constructions? 

I would focus on REALLY GOOD Realtor.